If you have access to the piping and it is PVC then it would be just as easy and a better job to install a new 90 degree elbow and flange at the 12 inch position. What is your situation as far as material and access? If your new toilet has a 12 inch rough in then you can get away with a little less than that sometimes so you may even be able to attach a flange to the existing elbow and "squeeze" the toilet in.